Battery chargers provide current-to charge-batteries in a wide variety of applications, such as in the field of communications, for charging batteries of portable radios. A battery charger generally includes a power supply which provides the charging current. One of the specifications of the power supply is power factor, which must be optimized to provide a desirable performance. In power supply terminology, power factor generally relates to the phase relationship of the source voltage and the harmonic current content. Ideally, a substantially in-phase relationship and a low harmonic current content is desired so that the power supply presents a substantially resistive load to the line source. Resistive loading of the line source prevents repetitive current spikes from being produced on the line source.
In conventional power supplies, a cyclically varying supply voltage is rectified by a rectifier, the output of which is coupled to a substantially large filtering capacitor in order to produce a rippled DC supply voltage. The DC voltage across the filtering capacitor is generally coupled to a regulator to provide a ripple-free DC voltage. Due to the filtering capacitor, high peak in-rush currents are produced when the power supply is turned on. These in-rush currents must be suppressed to avoid overstressing the power supply and the line input.
Additionally, the capacitive filtering of the rectified voltage substantially degrades the power factor of the power supply. In this arrangement, charging the filtering capacitor produces harmonic current spikes which must be suppressed or somehow controlled in order to improve performance of the power supply.
Several methods have been devised for controlling the in-rush currents. One such method comprises adding a series resistor which extends charge time constant of the filtering capacitor. However, this method significantly reduces efficiency of the supply and produces substantial heat which must be dissipated through elaborate packaging techniques. Alternatively, in order to improve efficiency, a thermistor can be used instead of the resistor to lower the resistance in accordance with the rise in temperature. However, upon occurrence of a power glitch or a fast on/off, this scheme may be ineffective when the thermistor is already heated up.
Switching power supplies generally use a pulse width modulator where the output voltage is regulated by varying pulse width or a duty cycle of the output signal. In some applications, the pulse width is varied by varying the switching rate of the pulse width modulator. In this arrangement, any changes in the input voltage or the output voltage cause an instantaneous change in switching duty cycle which causes instantaneous changes in the current being drawn by the supply. The instantaneous current change substantially increases the harmonic current contents and, therefore, degrades the power factor. Some switching supplies include a pre-switcher coupled between the rectifier and the capacitive filter to improve the power factor. However, addition of the pre-switcher significantly increases cost and complexity of the power supply.
Another method for controlling harmonic current spikes comprises adding a very large inductor to create an inductive input circuit. However, the inductor itself introduces a substantial phase shift which degrades the power factor.
It is, therefore, desired to provide a power supply which has a substantially improved power factor and avoids the expense and the complexity of prior art approaches.